Wireless communication networks and systems are growing both in the number of different services that are being provided and in the number of users using the different services. The increasing amount of users and an increasing amount of traffic in the wireless communication networks put increased demands for capacity and coverage on the networks and/or systems. One way of increasing capacity and coverage is to place more network nodes or radio base stations, RBSs, in the area covered by the wireless communication network.
More recent technologies utilise higher and higher frequencies, which experience increasing pathloss with increasing frequency. In addition, deployment in urban areas suffers from difficult propagation conditions. As a result of these factors, RBSs tend to be deployed in ever-tighter configurations, also possibly with varying output power, so-called heterogeneous network deployments.
A heterogeneous network is characterised by including several network nodes or RBSs with different transmit powers. Here the coverage areas of the low- and high-power RBSs may overlap, such that a User Equipment, UE, may receive higher downlink, DL, signal strength from the high-power RBS than from the low-power RBS, even if the UE is physically closer to the low-power RBS than to the high-power RBS. In such a situation, the UE also has a smaller pathloss to the low-power RBS than to the high-power RBS. The UE thus has its best downlink to the high-power node while the best uplink, UL, is to the low-power RBS.
The area in which the UE is physically present can thus be covered by several RBSs, which form individual cells, i.e. coverage areas of the RBSs. By periodically measuring a signal strength from several RBSs, the UE is enabled to select which RBS it should be connected to, or served by. The process of changing connected/serving RBSs for reception and transmission is referred to as a handover. Other forms of heterogeneous networks are the so called “shared cells” where low power RBSs do not constitute individual cells and they share the same cell-ID as the dominant macro RBS, i.e. the high-power RBS.
In a heterogeneous network scenario, it may be beneficial to associate the UL and the DL to different RBSs, so-called “UL/DL decoupling”. In a system that utilises coordinated multipoint, CoMP, transmission and/or reception this is further generalised in the sense that several transmission, Tx, points (i.e. physical antenna sites) and/or several reception, Rx, points are potentially used to serve a single UE. The Tx and Rx points need not be the same but the sets can (partially) overlap.
In the 3rd Generation Partnership Project, 3GPP Long Term Evolution, LTE, standard, the UE has the ability to transmit so-called Sounding Reference Signals, SRS, on the UL. These are used by the RBS to estimate a channel quality for a given UE, and the estimates are used by the RBS, or a scheduler, to place subsequent UL transmissions on the best possible part of the frequency band, also called frequency selective scheduling. In the context of UL/DL decoupling they could also be used to select which RBS(s) to associate the UL transmission to.
SRS may also be used in downlink for co-ordinated scheduling and transmission point selection. This is particularly feasible in a shared cell deployment where fast coordination is inherited from the architecture with the centralised common RBS, or eNB. Measurements done in downlink by the UE may be delayed by measurement reporting.
For Time Division LTE, TD-LTE, channel reciprocity can be exploited to perform beamforming in the downlink. Channel knowledge is obtained using uplink SRS.
SRS was initially introduced in the LTE standard with the purpose of allowing UL link adaptation. However, new deployments and technologies such as shared cell and CoMP may exploit UL measurements based on SRS for additional purposes, such as mobility and either transmit or receive point selection. Such new applications may introduce increased load on SRS, since more UEs than initially considered need to access the common SRS resource pool (at least in synchronised network deployments). Therefore, SRS capacity limitation may appear in new deployments.
Furthermore it becomes important to control SRS cost signalling overhead. It also delays the time to switch on SRS, alternatively requires larger margins. This can be crucial in dense networks with small cells and for mobiles with high speed.